The energetically favorable cis peptide bond for the azaglycine-containing peptide: For-AzGly-NH2 model

Abstract

The conformational preferences of the azaglycine-containing peptide model, For-AzGly-NH₂ (1), were investigated with ab initio and DFT methods for the cases when the formyl group has either a trans (1a) or cis (1b) peptide bond. Based on the HF/6-31G* potential energy surfaces, the minimum energy conformations for 1 were characterized. The structures and energetic relations between the resulting minima for 1 were systematically examined using various basis sets (6-31G*, 6-31G**, 6-311G** and 6-31 + G**). An important result, that the global minimum of 1b (ω₀≈0°) is more stable than that of 1a (ω₀≈180°), was found. This is comparable to the glycine peptide model, For-Gly-NH₂ (2), whose global minimum energy conformation prefers the trans peptide bond to the cis peptide bond as already known. The resulting minima for 1 demonstrate its utility as a valuable biological probe.